iPhone 11 Pro Max is estimated to cost $500 [1], but retails at $1100 for the 64GB model [2]. Based on their public numbers [3], the most you can get is $450, so they're still ahead if they offer a trade-in discount and just dispose of the old phone. More importantly, they either get to accelerate a potential sale (if the phone is still working) or they get to compete more aggressively on price with high-end android phones

[1] https://www.nbcnews.com/now/video/how-much-does-it-cost-to-m...

[2] https://www.apple.com/shop/buy-iphone/iphone-11-pro

[3] https://www.apple.com/shop/trade-in